- functions and operators generated by the compiler now use (almost) regular

- functions and operators generated by the compiler now use (almost) regular
dispatch functions of the form "visit_<opname>" and "visit_<funcname>_fn"
to provide customed processing.  This replaces the need to copy the "functions"
and "operators" dictionaries in compiler subclasses with straightforward
visitor methods, and also allows compiler subclasses complete control over
rendering, as the full _Function or _BinaryExpression object is passed in.
- move the pool assertion to be module-level, zoomark tests keep the connection
open across tests.